Add more samples after render
You can enable Progressive Refine (Render Properties &gt; Performance &gt; Tiles), set the sample count to a high amount like 10000 and stop the process manually (ESC) when satisfied.
However, I'd suggest use the Render Region feature and drag a rectangle around the darkest spot in the image (CtrlB), set the viewport samples to 0 and wait until the noise is resolved to get an idea how many samples are required and do a full render based on that knowledge.